Understanding Hydraulic Regulators: Types, Functions, and Design Procedure

 Lecture 16

Regulator

The water which enters into the main canal from the river has to be divided into different Branches and Distributaries, in accordance with the relative urgency of demand on different channels. This process of distribution is called 'Regulation'. To distribute water effectively, the discharge has to be adjusted to any desired value. This aim is achieved by means of regulators.

Canal Regulation Works

The works which are constructed in order to control and regulate discharges, depths, velocities etc. in canals, are known as canal -regulation works. These structures ensure the efficient functioning of a canal irrigation system, by giving full control upon the canals. The important of these structures are:

(i)               Canal Falls.

(ii)             Canal Regulators (Head Regulator and Cross Regulator).

(iii)          Canal Escapes.

(iv)           Metering Flumes, etc.

(v)             Canal Outlets and Modules.

 

Offtake alignment

When a distributary channel branches off from a parent channel, the offtake alignment must be carefully designed. The optimal alignment occurs when the offtaking channel initially aligns with the parent channel at zero angle and then gradually separates using transition curves (Fig. 1 a). These curves, applied to both channels, prevent silt accumulation and ensure even silt distribution. Without transition curves, both channels may form an angle upstream of the offtake (Fig. 1 b). If the parent channel must remain straight upstream and downstream, the offtake angle should be based on the channel edge, not the centerline (Fig. 1 c). However, the section should not be symmetrically narrowed, as an unbalanced offtake can create a silt jetty (Fig. 1 d), reducing the sectional area and causing bed scouring along the deviated flow.

 

Fig. 1 Offtake alignment

 

Cross regulators and distributary head regulators

Cross regulators and distributary head regulators are structures designed to manage and control the flow of water in canal systems. A cross regulator is installed on the parent channel downstream of the offtake point. Its primary function is to raise the water level upstream, ensuring that the offtaking channel can draw the necessary water supply. On the other hand, a distributary head regulator is positioned at the head of the offtaking channel (or distributary) to regulate the volume of water entering the offtaking channel. These structures work in tandem to maintain efficient water distribution between the parent channel and the offtaking channel.

Functions of Cross Regulators

1.     Cross regulators ensure efficient management of the entire canal network.

2.     When the water level in the main channel drops, they assist in raising it and supply the branching channels to meet their full requirements in a rotational manner.

3.     They allow the water supply to be cut off downstream in the main channel for repair or construction activities.

4.     Along with escape structures, they enable excess water to be released from the canals.

5.     They support transportation by allowing roads to be built over them with minimal additional expense.

6.     They help manage water level variations across different sections of the canal system, reducing the risk of breaches in the downstream areas.

7.     They regulate the flow of water where a canal discharges into another canal or a lake.

8.     Working in combination with falls, they help adjust the water surface slope to achieve the desired canal slope and cross-section.

Functions of Distributary Head Regulators

1.     They manage or adjust the flow of water from the main channel to the offtaking channel.

2.     They prevent silt from entering the offtaking channel.

3.     They act as a measuring device to determine the amount of water flowing into the offtaking channel.

4.     They allow for the cessation of water supply when it is unnecessary or when the offtaking channel needs to be closed for maintenance.

 

Design of Cross Regulator and Distributary Head Regulator

 

A.    Design of Crest and Waterway

 

Crest level:

The crest level of a cross regulator is typically set at the upstream bed level of the channel. In contrast, the crest level of a distributary head regulator is usually maintained 0.3 to 1.0 meters higher than that of the cross regulator. This difference ensures proper water distribution and control between the two structures.

              Length of waterway

The waterway length is calculated using the discharge formula for a drowned weir:

           Q = 2/3 C_d1 √(2g) L [(h+h_a)^3/2 – h_a^3/2] + C_d2 L d √(2g(h+h_a))

where

Q = discharge in cumec

L = length of clear waterway in m

h = difference in water levels on u/s and d/s of the crest in m

h_a = head due to velocity of approach

d = depth of d/s water level in the channel above the crest in m

g = acceleration due to gravity in m/s²

C_d1 = coefficient of discharge for freely discharging portion = 0.557, and

C_d2 = coefficient of discharge for submerged portion = 0.80.

        Generally, the head due to velocity of approach h_a being small is neglected.

B.     Design of Impervious Floor

Level and length of downstream floor:

The level and length of the downstream floor are determined under two conditions:

1.     Full Supply Discharge: Both the cross regulator and distributary head regulator operate with fully open gates.

2.     Insufficient Discharge in Parent Channel: The offtaking channel runs full at F.S.L., with the cross-regulator gates partially open to maintain flow.

 

Downstream floor level:

For both conditions, the discharge intensity qq and head loss H_L​(=h) are known. The corresponding value of Ef2​ (height of downstream T.E.L. above the downstream floor) is obtained from Blench curves. The downstream floor level is then determined using the relation:

d/s floor level = d/s T.E.L – E_f2 ≈ d/s F.S.L.– E_f2

In hydraulic design, the first flow condition typically governs, but sometimes the second condition is more critical due to a smaller discharge intensity (q) combined with a higher head loss (H_L). Even if the calculated downstream floor level for the worst condition is higher than the downstream bed level, the floor should always be set at or below the downstream bed level, never above it.

Length of downstream floor:

If E_f1 is height of upstream T.E.L. above the downstream floor, then

E_f1 = E_f2 + H_L

The depth D₁ and D₂ corresponding to E_f1 and E_f2 respectively are found from specific energy curves for different flow conditions.

Length of d/s floor = 5 (D₂ – D₁)

However, the length of the downstream floor should at least be equal to 2/3rd of the total length of the impervious floor.

Cutoff

Upstream cutoff:

The minimum depth of upstream cutoff below the stream floor level is given by

d₁ = 1/3 u/s water depth + 0.6 m

Downstream cutoff:

 The minimum depth of downstream cutoff below the downstream floor level is given by

d₂ = 1/2 d/s water depth + 0.6 m

Total Length of Impervious Floor

With a fixed downstream cutoff depth (d₂), the total length of the impervious floor (b) is determined based on the safe exit gradient (GE). The regulator floor experiences maximum static head when the channel is closed, and the upstream maintains full supply level to feed the offtaking channel.

Maximum static head

H_s = u/s F.S.L. – d/s floor level

G_E = H_s /d₂ * 1/(π√λ)

1/(π√λ) = G_Ed₂ / H_s

Knowing maximum static head H_s, downstream cutoff depth d₂, and safe exit gradient G_E, the value of 1/(π√λ) determined. Then from Khosla’s exit gradient curve for the value of 1/(π√λ), the value of α=b/d₂ is obtained.

The total length of impervious floor b is given by

b = αd₂

The required impervious floor length is provided on the downstream side, with the remaining length on the upstream side. The crest and downstream floor are connected by a 2:1 slope glacis, while in distributary head regulators, the crest and upstream floor are joined by a 1:1 slope glacis.

Thickness of Impervious Floor

The thickness of an impervious floor is determined by uplift pressure, with a minimum practical thickness of 0.3 to 0.5 m.

C.     Design of Upstream and Downstream Protection Works

The upstream scour depth d₁​ is calculated as 1/3​ of the upstream water depth plus 0.6 m, while the downstream scour depth d₂​ is 1/2​ of the downstream water depth plus 0.6 m.

Upstream Protection Works:

·         Block protection: d₁​ cubic meters per meter width.

·         Launching apron: 2.25d₁​ cubic meters per meter width.

Downstream Protection Works:

·         Inverted filter: d₂​ cubic meters per meter width.

·         Launching apron: 2.25d_2​ cubic meters per meter width.

D.    Devices to Control Silt Entry into The Offtaking Channel

To control silt entry into an offtaking channel, a raised crest at the distributary head regulator is used, but it is insufficient alone due to turbulence and silt accumulation. Additional devices are employed:

1.     King’s Vanes: Vertical, curved walls placed in the parent channel to deflect silt-laden bottom water away from the offtaking channel at a 30° angle. Made of R.C.C. or steel, they are 1/4th the water depth in height and spaced 1.5 times their height. They extend 0.6–1.5 m beyond a 2:1 inclined line from the offtake. Effective in silt control, but fail if turbulence is excessive.

2.     Gibb’s Groyne Wall: An extension of the downstream wing wall into the parent channel, dividing flow proportionally between the offtaking and downstream channels. It ensures silt is divided proportionally, preventing excessive silt entry. Adjustments can be made to reduce silt intake by allowing surplus flow through a hole in the wall.

3.     Cantilever Skimming Platform: A slab cantilevered into the parent channel below the crest level, separating top and bottom water to prevent silt from climbing into the offtaking channel. It disrupts silt ramp formation.

Blench curve

Energy flow curves–Montague curves